Conventional crystal modulated lasers, such as waveguide CO.sub.2 lasers, have an electro-optic crystal positioned within the laser's resonant cavity to frequency modulate the laser light produced by the laser gain medium. This is accomplished by applying a periodically changing voltage to the crystal. The periodically changing voltage causes the index of refraction of the crystal to change in synchronism with the modulation voltage. This changes the effective optical length of the laser resonator, causing the frequency of the laser light to be modulated.
The linear FM chirp laser, as disclosed in U.S. Pat. No. 4,666,295, which is incorporated by reference herein, is an example of a crystal modulated laser which produces a frequency-chirped output form the CO.sub.2 waveguide laser by applying voltage to an intra-cavity FM-cut CdTe electro-optic modulator. The applied voltage changes the refraction index of the crystal which, in turn, produces the FM chirp. This linear FM chirp laser system is deployed within a laser radar system to provide the system with the needed tuning range and linearity for pulse compression.
The frequency chirped output produced by the linear FM chirp systems have been showing large amounts of amplitude modulation. This is directly observable in the temporal output of such lasers and in their frequency spectrum. This phenomenon becomes more apparent in the higher gain laser systems.
AM modulation in the laser's output is caused by the loss and gain modulation driving relaxation oscillations in the resonator cavity. Relaxation oscillations are a natural damped oscillation in power that lasers exhibit if the cavity is disturbed. It is the result of the interaction of the circulating energy in the resonator with the gain of the CW pumped lasing medium.
Gain and loss modulation can be caused in a number of manners. For FM chirped lasers, the gain modulation is the direct result of the frequency chirp, since the laser gain is dependent on the optical frequency through the line shape of the gain medium. Loss modulation can be caused by stress birefringence in the electro-optic crystal, which will depend on the voltage applied to the crystal. Birefringence is a property of certain crystals characterized by a different index of refraction for different light polarizations. A highly birefringent crystal can rotate light from one polarization to a different polarization, producing a loss which reduces the laser efficiency. In the case of the linear FM chirp laser, a portion of the linearly polarized laser light within the resonant cavity becomes elliptically polarized upon passage through the FM-cut CdTe crystal. Since the elements within the resonant cavity are linearly polarization-sensitive, the elliptical polarization caused by the birefringence subjects the modulated output to an amplitude loss.
It is found in practice that these large amplitude variations cause a number of problems in the laser radar systems. For example, the variations make locking the laser frequency extremely difficult. The lock-loop within the system relies upon small amplitude changes caused by gain variations to hold the lazing frequency at line center. When the large amplitude variations caused by the crystal birefringence swampout the small amplitude changes relied upon by the lockloop, frequency control becomes difficult.
Most methods for correcting the amplitude variation in the modulated output are not practical. One method currently being used is to operate the laser while maintaining the crystal at a specific temperature. Since the birefringence within the crystal is generally unknown prior to laser operation, the procedure must be performed experimentally to determine the best working temperature. Through an exhaustive trial and error process, a very narrow temperature range, usually within a half degree, can be found in which the laser will operate with minimal amplitude fluctuation. Because this procedure is extremely time consuming and requires expensive electronic temperature feedback circuits to maintain the temperature within the required range, much of the amplitude variation goes uncorrected.
A system is disclosed in U.S. Pat. No. 5,018,153, entitled METHOD AND APPARATUS FOR CORRECTING AMPLITUDE VARIATION IN ELECTRO-OPTIC LASER SYSTEMS and assigned to the assignee of the present application, which improves the amplitude variation in the frequency modulated light produced by an electro-optic crystal laser system by compensating for the crystal birefringence within the resonant cavity. The resulting laser system can substantially reduce the amplitude dip in the modulated output without the difficulties involved in controlling the crystal's temperature.
This system although effective in some applications has limited range because it utilizes passive components. Accordingly in those systems where there are large amplitude variations on the order of 50% the above-identified system would not operate in an effective manner.
There is, therefore, a real need to provide an effective method and apparatus for correcting the losses within an electro-optic modulated laser system over a wide amplitude range. The present invention provides such a system.